I think you’re better off getting a pair of Sonos One SLs to be your surround speakers for the Arc, and place them ‘behind’ you, rather than in front of you in the corners where you have the Fives. That way, you can leave the Fives set up for music and your turntable’s input device, and still have an appropriate ‘surround’ setup with the speakers that are supposed to be ‘behind’ you as such. 

Slightly less expensive than buying a Port (no need for an Amp unless you’re driving another set of non-Sonos speakers) just for the line in use. 

Yes, you end up with two ‘rooms’ in the Sonos controller, but ultimately, I think you’ll be happier. FWIW, this is essentially the setup I have, although I no longer use a turntable, all my vinyl has long been digitized and lives on an NAS. 

As you laid out, there’s no way for a speaker setup as surround to send audio data (via aux input) back to the main soundbar.  There’s essentially three paths you can take to fix the problem.

One option is to buy a Sonos product with aux input and set it up as a separate room than your Arc+Sub+Fives.  That would be another Five, Amp, Port, or a Beam if bluetooth is an option.

Second option is to keep your Fives as they are for playing music, and use other speakers for surround duty.  A pair of Ones, One SLs, or symfonisk speakers would be just fine.  The downside to this is your sub can be bonded to the Arc or the Fives, not both, in this scenario.

A third option would be to run the audio from your turtable through the TV as an HDMI input somehow.  That way turntable audio comes through as TV audio.

A 4th option, if you want to consider it at all, is to just stop using the turntable and get that music through a streaming service, or a local library.

Thanks. These are both credible proposals, much appreciated. In both outcomes, I have to choose whether to connect my Sub to the TV or the turntable, which seems a shame.

I love my vinyl albums! I guess I will buy an Amp and send the signal to the Arc/Fives/Sub setup. I believe it has to be an Amp rather than a Port as there is no built-in preamp on the turntable.

Thanks all, DD.

It depends really.  If you setup one room as “2 Fives (turntable)” and the other as “Arc+Sub+surrounds” and group them together with turntable as the source, then you’ll bet the sub playing the turntable audio...along with Fives, arc, and whatever you use for surrounds.  Still not ideal.

This is incorrect.  The amp doesn’t have a phono connection and thus no builtin preamp for turntables.  None of the Sonos products do, including your Fives.  Either your turntable has a preamp and you’re not aware of it, or you’ve figured a way around it.  Either way, the Port will work just as good as your Five for bringing in the audio from your turntable.

@melvimbe  - thanks so much, you have just saved me the difference between a Port and an Amp. I do have a separate preamp but didn’t understand the difference until now. Cheers!

Can I ask a related scenario.
If the turntable was connected to a Port, would the surrounds playback as Full Music or as TV surrounds?

When set to "Full” in the Sonos app they will behave like that for al non-TV sound. 

Brilliant news. Phew. Thank you for the response